.TH std::ranges::prev 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::ranges::prev \- std::ranges::prev

.SH Synopsis
   Defined in header <iterator>
   Call signature
   template< std::bidirectional_iterator I >                      \fB(1)\fP \fI(since C++20)\fP
   constexpr I prev( I i );
   template< std::bidirectional_iterator I >                      \fB(2)\fP \fI(since C++20)\fP
   constexpr I prev( I i, std::iter_difference_t<I> n );
   template< std::bidirectional_iterator I >                      \fB(3)\fP \fI(since C++20)\fP
   constexpr I prev( I i, std::iter_difference_t<I> n, I bound );

   Return the n^th predecessor of iterator i.

   The function-like entities described on this page are niebloids, that is:

     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.

   In practice, they may be implemented as function objects, or with special compiler
   extensions.

.SH Parameters

   i     - an iterator
   n     - number of elements i should be descended
   bound - iterator denoting the beginning of the range i points to

.SH Return value

   1) The predecessor of i.
   2) The n^th predecessor of iterator i.
   3) The n^th predecessor of iterator i, or the first iterator that compares equal to
   bound, whichever is first.

.SH Complexity

   1) Constant.
   2,3) Constant if I models std::random_access_iterator<I>; otherwise linear.

.SH Possible implementation

   struct prev_fn
   {
       template<std::bidirectional_iterator I>
       constexpr I operator()(I i) const
       {
           --i;
           return i;
       }

       template<std::bidirectional_iterator I>
       constexpr I operator()(I i, std::iter_difference_t<I> n) const
       {
           ranges::advance(i, -n);
           return i;
       }

       template<std::bidirectional_iterator I>
       constexpr I operator()(I i, std::iter_difference_t<I> n, I bound) const
       {
           ranges::advance(i, -n, bound);
           return i;
       }
   };

   inline constexpr auto prev = prev_fn();

.SH Notes

   Although the expression --r.end() often compiles for containers, it is not
   guaranteed to do so: r.end() is an rvalue expression, and there is no iterator
   requirement that specifies that decrement of an rvalue is guaranteed to work. In
   particular, when iterators are implemented as pointers or its operator-- is
   lvalue-ref-qualified, --r.end() does not compile, while ranges::prev(r.end()) does.

   This is further exacerbated by ranges that do not model ranges::common_range. For
   example, for some underlying ranges, ranges::transform_view::end doesn't have the
   same return type as ranges::transform_view::begin, and so --r.end() won't compile.
   This isn't something that ranges::prev can aid with, but there are workarounds.

.SH Example


// Run this code

 #include <iostream>
 #include <iterator>
 #include <vector>

 int main()
 {
     std::vector<int> v{3, 1, 4};
     auto pv = std::ranges::prev(v.end(), 2);
     std::cout << *pv << '\\n';

     pv = std::ranges::prev(pv, 42, v.begin());
     std::cout << *pv << '\\n';
 }

.SH Output:

 1
 3

.SH See also

   ranges::next    increment an iterator by a given distance or to a bound
   (C++20)         (niebloid)
   ranges::advance advances an iterator by given distance or to a given bound
   (C++20)         (niebloid)
   prev            decrement an iterator
   \fI(C++11)\fP         \fI(function template)\fP
